[pashazanousi]

I simulated a floating roof of a tank with a rigid body motion, but I got this error and I don't know why the rigid body flies up?
The output displayed texts are followings:

COEFFICIENT LOOP ITERATION = 8 CPU SECONDS = 5.535E+01
----------------------------------------------------------------------
| Rigid Body Convergence |
+--------------------------------------------------------------------+
| | Quantity Change Rate |
+--------------------------------------------------------------------+
| Rigid Body 1 | Motion 1.085E+00 21.76 |
| | Force 1.039E+00 1.95 |
+--------------------------------------------------------------------+
----------------------------------------------------------------------
| SOLVING : Mesh Displacement |
----------------------------------------------------------------------
| Equation | Rate | RMS Res | Max Res | Linear Solution |
+----------------------+------+---------+---------+------------------+
| X-Disp | 0.00 | 0.0E+00 | 0.0E+00 | 0.0E+00 OK|
| Y-Disp | 0.53 | 1.1E-01 | 1.0E+00 | 4.1E-03 OK|
| Z-Disp | 0.00 | 0.0E+00 | 0.0E+00 | 9.1 0.0E+00 OK|
+----------------------+------+---------+---------+------------------+
| X-Disp | 0.00 | 0.0E+00 | 0.0E+00 | 0.0E+00 OK|
| Y-Disp | 0.01 | 5.7E-04 | 3.8E-03 | 9.3E-02 OK|
| Z-Disp | 0.00 | 0.0E+00 | 0.0E+00 | 29.4 0.0E+00 OK|
+----------------------+------+---------+---------+------------------+
| X-Disp | 0.00 | 0.0E+00 | 0.0E+00 | 0.0E+00 OK|
| Y-Disp | 0.07 | 3.9E-05 | 7.2E-04 | 9.1E-02 OK|
| Z-Disp | 0.00 | 0.0E+00 | 0.0E+00 | 41.5 0.0E+00 OK|
+----------------------+------+---------+---------+------------------+

+--------------------------------------------------------------------+
| ERROR #002100012 has occurred in subroutine Out_NegVol. |
| Message: |
| A negative ELEMENT volume has been detected. This is a fatal |
| error and execution will be terminated. The location of the first |
| negative volume is reported below. |
| Volume : -0.1433E-07 |
| Location : ( -0.14775E+02, 0.14609E+01, 0.50000E-02) |
+--------------------------------------------------------------------+

CFD Solver finished: Wed Feb 22 14:38:25 2023
CFD Solver wall clock seconds: 5.6570E+01

================================================== ====================
Termination and Interrupt Condition Summary
================================================== ====================

CFD Solver: The mesh is invalid or folded


+--------------------------------------------------------------------+
| Host Memory Information (Mbytes): Solver |
+--------------------------------------------------------------------+
| Host | System | Peak | % |
+-------------------------+----------------+----------------+--------+
| DESKTOP-OG7E4MI | 16301.22 | 833.83 | 5.12 |
+-------------------------+----------------+----------------+--------+

+--------------------------------------------------------------------+
| CPU Time Requirements of Solver |
+--------------------------------------------------------------------+

Subsystem Name Discretization Linear Solution
(secs. %total) (secs. %total)
----------------------------------------------------------------------
Mesh Displacement 9.16E+00 14.8 % 1.44E+01 23.2 %
Momentum and Mass 1.51E+01 24.4 % 5.94E+00 9.6 %
Volume Fractions 1.46E+00 2.4 % 9.61E-01 1.6 %
-------- ------- -------- ------
Subsystem Summary 2.58E+01 41.6 % 2.13E+01 34.4 %

GGI Intersection 1.17E-01 0.2 %
File Reading 1.13E+00 1.8 %
Variable Updates 3.45E+00 5.6 %
File Writing 4.59E+00 7.4 %
Miscellaneous 5.62E+00 9.1 %
--------
Total 6.20E+01

+--------------------------------------------------------------------+
| Job Information at End of Run |
+--------------------------------------------------------------------+

Host computer: DESKTOP-OG7E4MI (PID:6120)

Job finished: Wed Feb 22 14:38:28 2023

Total wall clock time: 6.171E+01 seconds
or: ( 0: 0: 1: 1.710 )
( Days: Hours: Minutes: Seconds )


+--------------------------------------------------------------------+
| An error has occurred in cfx5solve: |
| |
| The ANSYS CFX solver exited with return code 1. No results file |
| has been created. |
+--------------------------------------------------------------------+

End of solution stage.

+--------------------------------------------------------------------+
| The following transient and backup files written by the ANSYS CFX |
| solver have been saved in the directory D:/Third Paper/Floating |
| Roof-2D_pending/dp0_CFX_Solution/Fluid Flow CFX_001: |
| |
| 0_full.trn |
+--------------------------------------------------------------------+


+--------------------------------------------------------------------+
| Warning! |
| |
| The ANSYS CFX Solver has written a crash recovery file. This file |
| has been saved as D:/Third Paper/Floating |
| Roof-2D_pending/dp0_CFX_Solution/Fluid Flow CFX_001.res.err and |
| may be an aid to diagnosing the problem or restarting the run. |
| More details should be available in the solver output section of |
| the output file. |
+--------------------------------------------------------------------+


+--------------------------------------------------------------------+
| For CFX runs launched from Workbench, the final locations of |
| directories and files generated may differ from those shown. |
+--------------------------------------------------------------------+


This run of the ANSYS CFX Solver has finished.

[ghorrocks]

Rerun this simulation where you save a results file (including the mesh) at the time step before (or even better, every time step if you have the disk space available).

Look at the geometry just before it crashes and you will see where it goes bezerk. That is the first step in fixing the problem.

[pashazanousi]

Thanks for your reply;
I ran the simulation several times after modifying the mesh and other simulation parameters interfere solving process, including timestep and maximum coefficient factor but each time I got the same error. In the last run I change the multi-phase of air-water to air-air and I didn't get this error anymore. I don't understand and I don't know where that error came from?

Initial Fluid VF.png

Initial Local Pressure.png

Fluid VF res.err.png

Local Pressure res.err.png

[pashazanousi]

Below are some pictures that describe the rigid body settings I used in this simulation; I wonder if they will be helpful.

RB1.PNG

RB2.PNG

RB3.PNG

[ghorrocks]

You can see the mesh at the ends of your device is being stretched until it turns inside out. That is where your problem is.

You are going to have to modify your setup so this area of mesh does not get distorted like that.

[pashazanousi]

Thanks for you reply;
I know what you mean, but I have refined the mesh several times in the distorted region but I keep getting this error. Moreover, what caused the rigid body to fly up? That's not sensible at all. Would you mind sharing your thoughts on that?

[ghorrocks]

Refining the mesh is not how you fix it.

What caused it to fly up? You have done a modelling error. If you want us to help identify the error please post your output and an image showing your boundary patches.

How to avoid the mesh stretching? First of all fix the non-physical flying up issue, then we will look at whether you need to take a different approach of modelling this.

[pashazanousi]

Thanks for your reply;
Attached is the output file and some pictures that correlate with the boundary conditions I implemented in this simulation.
Boundary Condition.png

Subdomains.png

Output file.txt

[ghorrocks]

Have you done the floating buoy tutorial? There is a CFX tutorial which looks very similar to what you are doing. Here is a link to it: https://ansyshelp.ansys.com/account/...the_heatx.html

But note you will need a support license to access this link.

It is Chapter 30 in the CFX tutorial manual, "Modeling a Buoy using the CFX rigid body solver"

[pashazanousi]

Thanks for your reply;
I got the desired result from simulating a floating rigid body so far. In this specific case, I am struggling, and I don't know where the error came from? I used ANSYS Student 2023 R1, but I don't think that is the problem. Anyway, I'll review the tutorial again. Thank you so much for your generosity...

[ghorrocks]

I do not know what you are trying to do, so this is a guess: but try changing the mesh motion option on the side wall boundary conditions to "unspecified".